1. Field of the Invention
The present invention is directed to thermal control of spacecraft and/or instruments, and, more particularly, to the use of miniaturized mechanical louvers to control the temperature of a spacecraft and/or instruments.
2. Description of the Related Art
All spacecraft and the instruments they support require an effective thermal control mechanism in order to operate as designed and achieve their expected lifetimes. In an increasing number of satellites, optical alignment and calibration require a strict temperature control. Traditionally the thermal design, an iterative process, is part of the spacecraft design determined by all the subsystems and instruments. Heat load levels and their location on the spacecraft, equipment temperature tolerances, available power for heaters, view to space, and other such factors are critical to the design process. Smaller spacecraft with much shorter design cycles and fewer resources such as heater power, volume, and available radiator surface area, require a new, more active approach. The relatively low mass of smaller spacecraft result in larger temperature variations as loads and sink temperatures change, unless some means of modulating their heat rejection rate is provided.
A spacecraft must normally be within a reasonable thermal equilibrium. As the heat load and/or thermal sink changes, it is often necessary to modify the heat rejection rate in some controlled fashion.
A number of active methods which vary the heat rejection rate in a controlled fashion are commonly used to maintain a reasonable thermal equilibrium. One such method is to cold bias the spacecraft and use simple electrical resistance make-up heaters to control the temperature.
However, this can require considerable electrical power, which the spacecraft may not have available at all times.
Another approach is to employ a radiator connected to the heat dissipating equipment with variable conductive heat pipes, capillary pumped loops, or loop heat pipes. This approach is effective but adds weight, cost and complexity. In addition there are ground testing issues with heat pipes.
Yet another approach is to use mechanical louvers that can open to expose a radiative surface and close to hide it. Mechanical louvers have frequently been used for spacecraft and instrument thermal control purposes. These devices typically consist of parallel or radial vanes, which can be opened or closed to expose an underlying surface and thus vary the effective emissivity. While functional, traditional mechanical louvers are bulky, expensive, subject to damage, and require significant thermal analysis to evaluate the effect of different sun angles. Louver assemblies are explained in Satellite Thermal Control Handbook, David G. Gilmore (editor), The Aerospace Corporation Press, El Segundo, Calif. 1994, pp. 4-99 through 4-103.
In order to meet current and future space science goals, miniaturized spacecraft with greatly reduced size and mass, short design and build cycles, and restricted resources (power, command, control, etc.) are required. Spacecraft in this very small size range, 10 to 20 kg, will require smaller thermal control subsystems. Their low thermal capacitance will subject them to large temperature swings when either the heat generation rate or the thermal sink temperature changes. The ST5 Nanosat Constellation Trailblazer mission, for example, has a requirement to maintain thermal control through extended earth shadows, possibly over 2 hours long.
All spacecraft rely on radiative surfaces to dissipate waste heat. These radiators have special coatings, typically with low absorptivity and a high infrared emissivity, that are intended to optimize performance under the expected heat load and thermal sink environment. As discussed above, given the dynamics of the heat loads and thermal environment it is often necessary to have some means of regulating the heat rejection rate of the radiators in order to achieve proper thermal balance. The concept of using a specialized thermal control coating or surface which can passively or actively adjust its effective emissivity in response to such load/environmental sink variations is a very attractive solution to these design concerns. Such a system would allow intelligent control of the rate of heat loss from a radiator. Variable emissivity coatings offer an exciting alternative that is uniquely suitable for micro and nano spacecraft applications.
Variable emittance thermal control coatings change the effective infrared-red emissivity of a thermal control surface to allow the radiative heat transfer rate to be modulated upon command. Two known variable emittance thermal control technologies currently under development include electrochromic devices and electrophoretic devices. Both of these technologies are chemically based and are currently under development. Their suitability for the harshness of the space environment (i.e., degradation from radiation, atomic oxygen reactions, temperature extremes, etc.) is not yet established. In addition, reaction rates at cold temperatures may be an issue.
For electrochromic devices, the emittance modulation is achieved using crystalline or semi-crystalline electrochromic materials whose reflectance can be tuned over a broad wavelength (2 to 40 microns) in the infrared. The electrochromic process is a reversible, solid-state reduction-oxidation (redox) reaction. These materials become more reflective as the concentration of an inserted alkali metal (typically lithium) increases. This change is due to an increase in the electron free density, which causes the material to undergo a controlled transition between an IR transparent wide gap semiconductor and an IR reflective material. The electrochromic material is typically sandwiched between ITO electrical grids and is also in contact with an ion-conducting layer that contains the alkali metal. When a small bias voltage (typically +/xe2x88x921 VDC) is applied, the alkali ions shuttle to one side or the other, thus changing the effective emissivity of the surface.
Electrophoretic devices involve the movement of suspended particles (i.e., very small flakes) through a fluid under the application of a small electrical field. The particles carry electric charges that are acted upon by this field thus causing their movement through the fluid medium. This medium is highly absorptive. The particles are made of, or coated with, a material that has a high reflectivity. When an electric field is applied the flakes are attracted to the electrode and align themselves with their faces parallel to the surface, thus displacing the absorptive fluid medium. They overlap and form an essentially flat surface that is both a high reflector and spectrally reflective. When the electric field is reversed the flakes are drawn to the electrode on the other side of the highly absorbing fluid medium. The exposed surface thus becomes highly absorptive. This process has been demonstrated to be reversible and should be repeatable for thousands of cycles.
For the reasons stated above, both the conventional and emerging, chemically based variable emissivity technologies for thermal regulation of spacecraft have significant problems.
It is an object of the present invention to overcome the above-mentioned problems related to thermal regulation.
It is a further object of the present invention to provide thermal control for small, micro, or nano spacecraft.
It is another object of the invention to provide finer gradations in the regulation of effective emissivity and effective absorptivity of a surface, independent of sun angle.
Yet a further object of the invention is to provide a thermal control device rugged enough to withstand the demands of launch and spaceflight.
A micro-electromechanical device of the present invention comprises miniaturized mechanical louvers, referred to as Micro Electro-Mechanical Systems (MEMS) louvers of the present invention. The MEMS louvers of the present invention are another form of a variable emittance control coating and employ micro-electromechanical technology.
In a function similar to traditional, macroscopic thermal louvers, the MEMS louvers of the present invention change the emissivity of a surface. With the MEMS louvers of the present invention, as with the traditional, macroscopic louvers, a mechanical vane or window is opened and closed to allow an alterable radiative view to space.
Micro-machining techniques allow the generation of arrays of such MEMS louvers with feature sizes on the order of micrometers. This approach to variable emissivity control offers distinct advantages over traditional mechanical louvers with regard to size, weight, mechanical complexity, redundancy, and cost. In addition, the thermal analysis effort is simpler since the dependence on sun angle can be eliminated by having MEMS louvers face all directions. Moreover, the MEMS louvers of the present invention are more rugged, less costly, lighter, suitable for a wider range of applications, and are more specifically suitable for micro and nano spacecraft than the significantly larger, traditional louvers.
The above objects can be attained by a system that comprises the above-mentioned MEMS louvers of the present invention.
These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.